The light-emitting diode (LED) is a solid-state semiconductor light-emitting device resulting from joining a p-type semiconductor and an n-type semiconductor. Since the LED possesses strong points, such as long service life, excellent crashworthiness, low electric power consumption and high reliability, and as well enables decreasing size, thickness and weight thereof, it has been coming into use as light sources for various apparatus. Particularly, the white LED has been coming into use as disaster prevention lighting fixtures requiring reliability, in-vehicle lighting fixtures and liquid crystal backlights favoring decreases of size and weight, and railroad information boards for displaying train destinations necessitating visual recognition. It is also expected to find application to general household interior illuminations.
When the electric current is passed in the forward direction through the p-n junction formed of a direct transition semiconductor, the resultant recombination of electrons and holes causes the p-n junction to emit light having a peak wavelength conforming to the forbidden bandwidth of the semiconductor. Since the emission spectrum of the LED generally has a narrow half-value width of the peak wavelength, the color of the emission of the white LED is exclusively obtained in accordance with the principle regarding the mixed of colors of lights.
Then, the EL means the emission of light that is induced by the excitation of an electric field. Since the EL lamp emits homogenous light irrespective of the angle of vision and exhibits excellent crashworthiness, it is expected to find growing application to the field of platform panel displays in portable telephones and personal computers and the field of ordinary lighting fixtures characterized by surface emission of light.
Now, by reference to a white LED, a method for deriving a white color from the LED will be described specifically below. Also for the EL, the same method is used.
The known methods include (1) a method that combines three kinds of LEDs emitting a red color (R), a green color (G) and a blue color (B), respectively, and mixes these LED lights, (2) a method that combines an ultraviolet LED emitting an ultraviolet ray and three kinds of phosphors respectively emitting fluorescences of R, G and B in consequence of the excitation by the ultraviolet ray and mixes the fluorescences of the three colors emitted by the phosphors and (3) a method that combines a blue LED emitting a blue light and a phosphor emitting a fluorescence of an yellow color having the relation of an additive complementary color with the blue light in consequence of the excitation by the blue light and mixes the blue LED light and the yellow light emitted from the phosphor.
The method for obtaining a prescribed emission color using a plurality of LEDs necessitates a special circuit adapted to adjust the electric currents of the individual LEDs for the purpose of balancing the different colors. In contrast, the method for obtaining a prescribed emission color combining an LED and a phosphor is at an advantage in obviating the necessity for such a circuit and lowering the cost of the LED. Thus, various proposals have been made heretofore with respect to the phosphors of the kind having an LED as a light source.
For example, the YAG phosphor resulting from doping with Ce a YAG oxide host crystal represented by the composition formula of (Y, Gd)3(Al, Ga)5O12 has been disclosed (refer to Takashi Mukai et al., Applied Physics, Vol. 68, No. 2 (1999), pp. 152-155). In this document is described the point that the coating of the surface of an InGaN-based blue LED chip with a thin layer of the YAG phosphor results in mixing the blue light emitted from the blue LED and the fluorescence having a peak wavelength of 550 nm and emitted from the YAG phosphor in consequence of the excitation by the blue light and giving rise to a white light.
Further, the white LED resulting from combining a light-emitting device, such as a nitride-based compound semiconductor capable of emitting an ultraviolet ray, and a phosphor emitting light in consequence of the excitation by the ultraviolet ray has been disclosed. As phosphors usable herein, (Sr, Ca, Ba)10(PO4)6Cl2:Eu emitting a blue light, 3(Ba, Mg, Mn)0.8Al2O3:Eu a green light and Y2O2S:Eu a red light are also disclosed (refer to JP-A 2002-203991). An α-sialon-based phosphor has been proposed (refer to Japanese Patent No. 3668770). Here, the α-sialon has Ca or Y incorporated therein in the form of a solid solution.
The YAG-based phosphors are generally at a disadvantage in conspicuously decreasing the spectral intensity when the excited wavelength exceeds the near ultraviolet region.
Then, the white LED obtained by coating the chip surface of the InGaN-based blue LED with a phosphor formed of a YAG-based oxide is reputed to incur difficulty in acquiring high luminance because the excitation energy of the YAG-based oxide which is a phosphor and the excitation energy of the blue LED as the light source do not accord and the excitation energy is not efficiently converted.
Further, when the white LED is formed by combining a light-emitting device, such as the nitride-based compound semiconductor capable of emitting an ultraviolet ray, and the phosphor excited by the ultraviolet ray and enabled to emit light, it is held that this white LED incurs difficulty in acquiring a white color of high luminance on account of the problem that the mixing ratio of the phosphor as the red color component increases because the luminous efficiency thereof is considerably low as compared with the other phosphor.
An object of the invention is to provide a phosphor, with the α-sialon-based phosphor further developed and advanced and provide a light-emitting device using the phosphor, in which the phosphor is capable of imparting increased luminance to a white LED having a blue LED or an ultraviolet LED as a light source or realizing while EL having an EL light-emitting device as a light source.
The present inventor has pursued a diligent study with a view to accomplishing the above object and has perfected this invention consequently.